Reducing pollution and the consumption of water and energy are key environmental objectives for the petrochemical industry. Petrochemical production involves complex interdependences between water, energy, and environmental pollutant (WEE) systems, which limit the applicability of conventional methods for evaluating the systems individually. This study develops an integrated analysis of a system model, material and energy flow, and matrix to quantify the coupled relationships in the petrochemical WEE nexus (WEEN), and thus identify potential improvements bringing benefits to the overall WEEN at system, material and energy, and unit levels. It finds significant coupling and ripple effects in the petrochemical WEEN (with the energy system being dominant), three typical coupling types for materials and energy that lead to trade-offs between source reduction (dominated by input) and end-treatment (multi-element constraints), and three levels of coupling among the units (with the strongest having greatest potential for overall water and energy saving, and pollutant reduction). High resource consumption does not necessarily mean strong coupling or high potential improvements; adjusting a unit with low theoretical energy-saving potential can facilitate significant system optimization. This study therefore highlights the importance of an integrated "system of systems" perspective for industrial WEE management.